Cleaning composition for aluminum and aluminum alloys



Patented Dec. 31, 1946 CLEANING COMPOSITION FOR ALUMINUM AND ALUMINUM ALLOYS Lowell a. McCoy, Detroit, Mich., assignor to Wyandotte Chemicals Corporation, Wyandotte, Mich., a corporation of Michigan No Drawing. Application April 13, 1944, Serial No. 530,939

It is an object of this invention to prepare a cleaning composition in dry or substantially dry form, which when placed in solution will afford an excellent cleaner for aluminum and its alloys, preparatory to spot welding. While the composition inherently has some cleaning ability for various metals, its primary utility is for the pur-- pose specified.

It is also an object to prepare a cleaning bath andto employ a method wherein this composition is employed.

It is necessary to remove the oxide films formed during natural oxidation and heat treatment of aluminum and its alloys, which films are nonuniform and of high resistance in relation to that of the metal. It is now. quite generally agreed that in spot welding of aluminum alloys for aircraft structures the surface film plays an extremely important part in determining weld quality. One authority has stated, it appears that at least 75 per cent of the troubles experienced with surface spitting, weld spitting, irregularity in weld shape and the extent of electrode pickup can be attributed to improper surface preparation." Various methods both chemical and mechanicalhave been employed for removing oxide films from aluminum surfaces preparatory to spot welding. Each, however, leaves much to be desired.

Most of the oxide-removing solutions in use today fall into three categories: (1) hot or cold dilute acids (2) cold concentrated acids and (3) alkaline compositions. Hydroiluoric acid which is an example of (1) requires time control within a few seconds. Otherwise formation of film due to chemical reaction will result upon overtreatment, which film is injurious to good welding. Acids like phosphoric take too long a time unless traces of fluorides are present. The concentration of the latter is, however, almost impossible to control by any simple method. The alkaline type is not satisfactory except as degreasers because the strong alkalies attack aluminum and give highly erratic results.

Hydrofluosilicic acid, i. e. HzSiFe, which can be purchased commercially at a low cost in concentrations of 27-30% has been employed at about 3 and 4% by volume concentrations in baths, preferably in about 3%. The pH value based thereon is kept between 0.95 and 0.90 during the entire treating time of 6 to minutes at room temperature. This method using this acid has several advantages as is readily apparent from the above statements. Also there is less danger of overtreatment as is the case with most acids. However, there are disadvantages with all liouid acids such as the difllculty of handling, weight of inert solvent therein, cost of containers and chances of breaking them.

A mixture of ammorium silico fluoride and 8 Claims. (Cl. 252-142) sodium acid sulfate, 1. e. niter cake, when placed in an aqueous solution, I have found, liberates free fiuosllicic acid. But this solution possesses the disadvantage that sodium silico fluoride is precipitated in dilute solution, such as a 3% solution of said components in stoichiometricproportions. By the addition of ammonium sulfate, I have determined that all but a negligible part of this precipitation is prevented. The addition of a wetting agent such as an alkyl-aryl sodium sulfonate in small amounts promotes both rapidity and effectiveness of the action of the leaner. Where a wetting agent is not employedQtheresistance of the treated surface is higher and more difficult of duplication than in the case where it is present under the same condition.

Various wetting agents maybe employed. such as thealkali metal or basic nitrogen saltso'f sulfonated oils, sulfonated primary andsecondary alcohols. sulfated amides,'sulfated esters of mono-acid dibasic acids and higher alkyl-aryl sulfonates. The last type does not hydrolyze easily and is, therefore, stable in acid solutions. My preferred composition is a mixture of 18.5% (all percentages hereinafter given being by weight unless otherwise specified) ammonium silicofluoride, 25% niter cake, 52.5-54.5% ammonium sulfate and 2-4% of a higher alkyl-aryl sodium sulfonate, such as a kerosene -ben'zene sodium sulfonate. Niter cake is a crude sodium sulfate containing some bisulfate and very small amounts of other substances. But pure sodium acid sulfate will serve the purpose. vIn this mixture the first two recited components are in approximately stoichiometric proportions. That is to say, the ammonium silicofluoride and sodium sulfate are present in such proportions, based on their respective molecular weights, as to react according to the following chemical equation, producing fluosilicic acid:

A suitable concentration is 8 ounces of the com"- position to a gallon of water. The following table sets out times of treatment in the bat'h'of said solution at various temperatures.

Alclad 24 S-T is an alloy consisting of a base of aluminum alloy having good physical properties protected from corrosion by a lever of very pure aluminum on both surfaces of the sheet, the cladding constituting 15-10% of th thickness of the sheet. The corngosfltlfil oi the base metal is 93.7% A], 4.2% Cu, 0.6% Mn and Such treating times remain as indicated until the concentration (determined by the titration using N/l NaOH and methyl orange as indicator) falls to 1.5 gm/lOO cc.

The advantage of using this aforesaid ratio of constituents in the dry mixture is that the same proportions can be added for repleting. The concentration of the solution can be varied wide- 1y without much change of time treatment.

The concentration of the treating bath can vary from substantially 1.5% up to depending somewhat on the temperature; but for high temperatures the upper limit should not exceed 8%.

There are various methods in which this mixture can be employed in solution. By one mode aluminum objects are immersed in a solution at room'temperature containing approximately 6% by weightof the mixture for varying lengths of time depending upon the thickness of the metal as shown in the table. The reason for the effect of thickness of the metal upon the time required for treatment may be the presence of the thicker coating'of o'xideupon the thicker gages, due to longer time required for heat treatment. Or it may be due to the greater amount of copper in the cladding of the thinner metal due to the shorter distance through which it must diffuse from the base metal to the surface of the cladding. Traces of copper accelerate the action of the acid upon the aluminum. It may be both factors are responsible. Immediately following this treatment the objects are thoroughly rinsed and dried.

In laboratory tests, the aluminum so treated has been found to be suitable for spot welding after as much as a week has elapsed since said treatment. A longer period results in a change of the surface believed to be the result of oxidation. In factory practice, however, where chances of the object being contaminated through handling, etc., are much greater, a twenty-four hour limit between treatment and spot welding is to be preferred.

The above description and examples are illustrative only. Any modification of or variation therefrom which conforms to the spirit of the invention is intended to be included within the scope of the claims.

I claim:

1. A metal cleaning composition, particularly adaptable for removing oxide films from aluminum and its alloys, comprising ammonium silicefluoride and sodium acid sulfateboth present in substantially stoichiometric proportions for reaction with each other, and ammonium sulfate -a sulfonated'organic compound wetting agent.

3. In the process of removing oxide films from aluminum and its alloys, the step of applying to the surface thereof an aqueous solution comprising ammonium, silicofluoride and sodium acid sulfate both present in stoichiometric proportions for reaction with each other, water, and ammonium sulfate present in sufficient amount to inhibit the formation of a precipitate in said solution.

4. In the process of removing oxide films from aluminum and its alloys the step of applying to the surface thereof an aqueous solution comprising water and a mixture of, by weight 18% ammonium silicofluoride, 25% sodium acid sulfate and 52.5-54.5% ammonium sulfate.

5. In the process of removing oxide films from aluminum and its alloys, the step of applying to the surface thereof an aqueous solution comprising water and a mixture of, by weight 18% ammonium silicofluoride, 25% sodium acid suifate, 52.5-54.5% ammonium sulfate, and the balance a sulfonated organic compound wetting a ent.

6. A metal cleaning solution comprising water and a cleaning composition forming 1-10% by weight thereof and comprising ammonium silico-fiuoride and sodium acid sulfate both present in substantially stoichiometric proportions for reaction with each other, and ammonium sulfate present in an amount suflicient to inhibit the formation of a precipitate in said solution.

7. A metal cleaning solution comprising by weight a 1-10% aqueous solution of 18% ammonium silico fluoride, 25% sodium acid sulfate. 52.5-54.5% ammonium sulfate, and the balance a sulfonated organic compound wetting agent.

8. The process of cleaning aluminum and aluminum alloys which comprises treating the surface thereof with a 1-10% by weight aqueous solution of a mixture of 18% ammonium silicofluoride, 25% sodium acid sulfate and 52.5-54.5% ammonium sulfate, at a temperature of -90 F. and for a period of 4 to 10 minutes.

1.0m R. McCOY. 

